An electron multiplier is a common device component which uses secondary electron emission (SEE) to provide a gain in the intensity of incident radiation. In some device embodiments, for example, incident primary electrons pass a ‘window’ component of the device and scatter with a detector material capable of inducing a cascade of secondary electrons. By proper selection of the composition and physical state of the detector gain material, a gain is achieved when the yield of SEE is greater than one (i.e., on average more than one secondary electron is generated from each primary electron via inelastic scattering). In some detector schemes, for example, a plurality of electrodes capable of providing secondary electron emission, called dynodes, are provided in a stacked configuration. In these electron multiplier devices secondary electrons are generated upon interaction of radiation with a first dynode provided in the series. Secondary electrons from the first dynode are subsequently collected and accelerated toward subsequent dynodes provide in the series, wherein each dynode provides successive additional secondary electron emission. Secondary electron emission in these systems can be significantly enhanced using field emission (FE), wherein an electrical bias is provided to the system to facilitate extraction of the SEE generated. Electron multipliers are currently available that are capable of providing very significant gains functionality on the order of 105 to 109 for stack configurations.
Given their usefulness for amplifying electron signals, electron multiplier devices are key components in a range of systems including detectors, display devices and other high speed electronic systems. The application of electron multipliers for detector applications, for example, has led to the development of microchannel plate detector systems which are currently the most widely implemented detector platform for mass spectrometry. Wide spread use of these device components provides a motivation for the development of new materials and device configurations to improve the performance capabilities (e.g., gain, stability, lifetime etc.) and develop low cost fabrication pathways for electron multiplier devices.
Thin semiconductor membranes have been applied for quite some time as substrates for high-speed electronic devices, for display technology, as micromechanical devices such as pressure sensors, as mask materials for electron projection lithography, and as radiation detectors. These structures are particularly useful in device configurations wherein they provide a “window” for separating device components requiring specific, preselected operating conditions. In some detector systems, for example, semiconductor membranes provide a useful interface functionality for separating a detection environment from detector components that operate under vacuum and/or low temperatures conditions. Given the window functionality of thin semiconductor membranes, there is currently motivation to implement these device structures for a variety detector applications. The development of new semiconductor membrane structures capable of functioning as electron multiplier devices is expect to continue to enhance the utility of these structures in advanced detector systems.
A variety of semiconductors and semiconductor heterostructures are known to provide effective secondary electron emission upon exposure to radiation. Reducing the thickness of these materials to micron or submicron scales to achieve a semiconductor membrane configuration, however, substantially reduces secondary electron emission in many of these systems if the thickness corresponds to the inelastic mean free path of incident electrons. However, most common semiconductors have SEE yield below three, making it impractical to achieve a high gain be integrating multiple stacks. Providing some semiconductor materials in a semiconductor membrane configuration, for example, results in a complete loss of gain functionality. Hence, integrating a thin semiconductor membrane as a secondary electron emission element in a detector is currently not feasible for a range of important applications.
U.S. Pat. No. 4,060,823 provides electron emissive semiconductor devices consisting of separate regions of semiconductor materials spaced apart from each other by barrier device elements. Barriers for the disclosed electron emissive semiconductor devices include high resistance or insulating materials or alternatively p-n junctions capable of inhibiting or reducing current flow between the separated semiconductor regions. Device configurations using a thin membrane format are disclosed. The reference describes certain benefits achieved by the disclosed device configurations including protection against excessive electron emission currents, and a reduction in image spreading. Use of the electron emissive semiconductor devices as photocathodes and electron multipliers is disclosed.
U.S. Pat. Nos. 4,303,930, 5,138,402, and 5,814,832 describe semiconductor-based electron emitting devices having a multilayer configuration comprising a plurality of p-type and n-type doped semiconductor layers. In U.S. Pat. No. 4,303,930 the disclosed multilayer configuration has p-type semiconductor layers and n-type semiconductor layers integrated so as to generate a plurality of diode structures. Application of reverse bias voltage to the electrodes of the diode structures is reported to cause avalanche amplification and electron emission from the surface of the n-type layers. In U.S. Pat. No. 5,138,402, and 5,814,832 the disclosed multilayer configuration has a Schottky electrode and a p-type semiconductor layer, wherein electrons are emitted from the Schottky electrode in response to the application of reverse bias voltage. The disclosed multilayer structures are reported to provide stable device performance for a useful range of operating conditions.
It will be appreciated from the foregoing that electron emissive systems, such as electron multipliers and secondary electron emission systems, are needed for a range of applications including radiation detection, high speed electronics and display device applications. Particularly, thin semiconductor membrane-based electron emissive systems are needed that are capable of providing device interface functionality in addition to useful gain and bandwidth characteristics.